Radiation-sensitive mixture

ABSTRACT

A radiation-sensitive mixture, essentially consisting of 
     (a) a binder or binder mixture which is insoluble in water but soluble in aqueous alkaline solutions, 
     (b) a compound which forms a strong acid on exposure to radiation and 
     (c) one or more organic compounds which suppress the solubility of (a) in aqueous alkaline solutions, 
     wherein component (c) is an organic compound of the general formula (I) ##STR1## where R 1  and R 2  are each hydrogen, alkyl, alkoxy, aryl or aralkyl and R 3  and R 4  are each alkyl, cycloalkyl, aralkyl, aryl or a radical ##STR2## where n is from 1 to 5 and R 5  is alkyl, alkoxy, aryl or aralkyl, or R 3  together with R 4  form a five-membered to seven-membered ring via --(CH 2 ) m  -- in which m is from 4 to 6, is suitable for the production of relief structures.

The present invention relates to positive-working radiation-sensitivemixtures which contain a binder which is insoluble in water but solublein aqueous alkaline solutions, a compound which forms an acid whenexposed to radiation and an organic compound which undergoesacid-catalyzed hydrolysis, with the result that the alkali solubility ofthe mixture is increased. These mixtures are sensitive to UV radiation,electron beams and X-rays and are particularly suitable as resistmaterial.

Positive-working radiation-sensitive mixtures are known, and inparticular positive-working resist materials which containo-quinonediazides in binders which are soluble in aqueous alkalinesolutions, for example novolaks or poly-p-vinylphenols, are usedcommercially. However, the sensitivity of some of these systems toradiation, in particular short-wavelength radiation, is unsatisfactory.

Increases in the sensitivity of radiationsensitive systems which, in theprimary photochemical reaction, produce a species which then initiates acatalytic secondary reaction independently of the radiation have beendescribed. Thus, U.S. Pat. No. 3,915,706 describes, for example,photoinitiators which produce a strong acid which then cleavesacid-labile groups, such as polyaldehyde groups, in a secondaryreaction.

Other known radiation-sensitive mixtures are those based onacid-cleavable compounds which contain, as a binder, a polymer which issoluble in aqueous alkaline solutions, and a compound which forms astrong acid in a photochemical reaction and a further compound which hasacid-cleavable bonds and whose solubility in an alkaline developerincreases due to the action of the acid (cf. DE-A 3 406 927). Compoundswhich form a strong acid in a photochemical reaction are diazonium,phosphonium, sulfonium, iodonium and halogen compounds. The use of theseonium salts as photochemical acid donors in resist material is is alsodisclosed in, for example, U.S. Pat. No. 4,491,628. An overview of theuse of onium salts in resist materials is given by Crivello in Org.Coatings and Appl. Polym. Sci. 48 (1985), 65-69.

Radiation-sensitive mixtures of polymers having acid-labile side groupsand photochemical acid donors are disclosed in, for example, U.S. Pat.No. 4,491,628 and FR-A 2,570,844. However, these polymeric binders arehydrophobic and become alkali-soluble only after exposure.

Copolymers having phenolic and acid-labile groups, for examplepoly-(p-hydroxystryene-co-tertbutoxycarbonyloxystyrene), are disclosedin J. Polym. Sci., Part A, Polym. Chem. Ed. 24 (1986), 2971-1980.However, if those copolymers of this group which are stillalkali-soluble are used in conjunction with the commercial sulfoniumsalts, such as triphenylsulfonium hexafluoroarsenate, as also describedin U.S. Pat. No. 4,491,628, these mixtures have the disadvantage that avery large amount of material is removed from the unexposed parts, sincethe stated sulfonium salts do not sufficiently contribute towardsuppressing solubility. Furthermore, after exposure, these salts do nothelp to increase the solubility of the exposed parts in alkalinedevelopers, since no alkali-soluble photochemical products are formed.

DE-A 37 21 741 describes radiation-sensitive mixtures which contain apolymeric binder which is soluble in aqueous alkaline solutions, and anorganic compound whose solubility in an aqueous alkaline developer isincreased by the action of an acid and which contains one or moreacid-cleavable groups, this organic compound producing a strong acid onexposure to radiation.

It is an object of the present invention to provide novel highly activepositive-working radiation-sensitive systems for the production ofrelief structures, which systems can be developed with aqueous alkalinesolutions and permit the production of layers sensitive toshort-wavelength UV light.

We have found that this object is achieved if a radiation-sensitivemixture which contains a binder which is insoluble in water but solublein aqueous alkaline solution, an acid-labile organic compound which ishydrolyzed by the action of an acid and a compound which forms a strongacid on exposure to radiation is exposed imagewise and heated and theimagewise exposed parts of the layer are washed out with a developer.

We have found, surprisingly, that highly active radiation-sensitivesystems for the production of relief structures in short-wavelength UVlight are obtained by the use of certain pyrocatechol derivatives ascompounds which suppress the solubility of the binders in aqueousalkaline solutions, said systems having very good reproducibility andhigh resolution, especially in combination with novolak-based binders.

The present invention relates to a radiationsensitive mixture,essentially consisting of

(a) a binder or binder mixture which is insoluble in water but solublein aqueous alkaline solutions,

(b) a compound which forms a strong acid on exposure to radiation and

(c) one or more organic compounds which suppress the solubility of (a)in aqueous alkaline solutions,

wherein the organic compound (c) is a compound of the general formula(I) ##STR3## where R¹ and R² are identical or different and are eachhydrogen, alkyl, alkoxy, aryl or aralkyl, R³ and R⁴ are identical ordifferent and are each alkyl, cycloalkyl, aralkyl, unsubstituted oralkyl- or halogen-substituted aryl or a radical ##STR4## where n is from1 to 5 and Rs is alkyl, alkoxy, aryl or aralkyl, or R³ together with Rforms a five-membered to seven-membered ring via --(CH₂)_(m) -- in whichm is from 4 to 6.

The novel radiation-sensitive mixture may contain, as binder (a), aphenolic resin, for example a novolak having a mean molecular weightM_(w) of from 300 to 20,000 or poly-(p-hydroxystyrene),poly-(p-hydroxy-α-methylstyrene) or copolymers of p-hydroxystyrene andp-tert-butoxycarbonyloxystyrene, of p-hydroxystyrene and alkoxystyreneor of p-hydroxystyrene and 2-tetrahydropyranyloxystyrene (where thesecopolymers can also be prepared by a polymer-analogous reaction), havingmean molecular weights M_(w) from 200 to 200,000 in each case.

The binder component (a) is present in general in an amount of from 50to 95% by weight, based on the total amount of components (a)+(b)+(c),with the proviso that the sum of the percentages stated under (a), (b)and (c) is 100.

Preferred compounds (b) which form an acid on exposure to radiation areonium salts of the general formula (II) or (III) ##STR5## where R.sup.α,R.sup.β , and R.sup.γ are identical or different and are each alkyl,aryl, aralkyl or a radical ##STR6## where R.sup.δ, R.sup.ε and R areidentical or different and are each H, OH, halogen, alkyl or alkoxy andX.sup.⊖ is AsF₅.sup.⊖, SbF₆.sup.⊖, PF₆.sup.⊖, BF₄.sup.⊖, ClO₄.sup.⊖, CH₃SO₃.sup.⊖ or CF₃ SO₃.sup.⊖.

Component (b) is present in general in an amount of from 1 to 20% byweight, based on the total amount of components (a)+(b)+(c).

The novel radiation-sensitive mixture preferably contains, as theorganic compound (a) which suppresses the solubility of component (c) inaqueous alkaline solutions, one or more of the compounds ##STR7## whereR' and R" are identical or different and are each alkyl of 1 to 6 carbonatoms, phenyl or benzyl and n is 1, component (c) being present ingeneral in an amount of from 1 to 49% by weight based on the totalamount of components (a)+(b)+(c). The sum of the percentages statedunder (a), (b) and (c) is 100.

The novel radiation-sensitive mixture may additionally contain asensitizer which absorbs radiation and transfers it to component (b), oradditionally not more than 1% by weight, based on the total amount ofcomponents (a)+(b)+(c), of an adhesion promoter, a surfactant or a dye.

The present invention furthermore relates to a process for theproduction of relief structures or for structuring wafers by applicationof a photoresist solution in a layer thickness of from 0.1 to 5 μm to asubstrate pretreated in a conventional manner, drying, imagewiseexposure, if necessary heating to temperatures of up to 150° C anddevelopment with an aqueous alkaline solution, wherein the photoresistsolution used contains the novel radiation-sensitive mixture.

The novel system is particularly advantageous since it can be used incombination with the especially economical novolak-based binders. Therelief structures obtained with said system have very goodreproducibility and high resolution.

Regarding the components of the novel radiationsensitive mixtures, thefollowing may be stated specifically.

a) Because of the plasma etching stability generally required, suitablebinders or binder mixtures which are insoluble in water but soluble inaqueous alkaline solutions are in general phenolic resins, for examplenovolaks having molecular weights M_(w) of from 300 to 20,000,preferably from 300 to 2,000, g/mol, and, for exposure in theshort-wavelength UV range (≦300 nm), in particular novolaks based onp-cresol/formaldehyde, poly-(p-hydroxystyrenes) andpoly-(p-hydroxy-α-methylstyrenes), where these poly-(p-hydroxystyrenes)generally have molecular weights M_(w) of from 200 to 200,000,preferably from 1,000 to 40,000, g/mol and may furthermore be modifiedin a known manner by reaction (polymer-analogous reaction) of theirhydroxyl groups with, for example, chloroacetic acid, a chloroaceticester, an alkyl halide, a benzyl halide, 3,4-dihydropyran, dihydrofuran,a chlorocarbonic ester and/or a pyrocarbonic ester. The modifiedpolymeric binders (a) which are obtainable in this manner and in thepresent case are also understood as being copolymers are, for example,those obtained from p-hydroxystyrene andp-tert-butoxycarbonyloxystyrene, p-hydroxystyrene and alkoxystyrene andp-hydroxystyrene and 2-tetrahydropyranylhydroxystyrene. Preferredcopolymers of p-hydroxystyrene, which contain protective side groups,essentially possess the groups ##STR8## where x≧y and ##STR9## i.e.copolymers having p-(2-tetrahydropyranyl)-oxystyrene,p-(isopropyloxycarbonyl)-oxystyrene,p-(cyclohexyloxycarbonyl)-oxystyrene, p-(tert-butoxycarbonyl)-oxystyreneand/or p-(tert-pentyloxycarbonyl)-oxystyrene units.

Mixtures of the abovementioned binders (a) are also suitable. Binder (a)is present in the novel mixture in general in amounts of from 50 to 95,preferably from 70 to 85, % by weight, based on the total amount of theradiation-sensitive mixture (a)+(b)+(c).

b) Suitable compounds (b) which form a strong acid on exposure toradiation are in principle all compounds which have this property andthus act as an acid donor. However, iodonium and in particular sulfoniumsalts are preferred for exposure to short-wavelength UV radiation. Theseare of the general formulae (II) and (III) ##STR10## where R.sup.α,R.sup.β and R.sup.γ are identical or different and are each alkyl, forexample of 1 to 5, preferably 1 or 2, carbon atoms, aryl, e.g. phenyl,aralkyl, e.g. benzyl, or a radical ##STR11## where R.sup.δ, R.sup.ε andR are identical or different and are each hydrogen, OH, halogen, e.g.chlorine or bromine, alkyl, for example of 1 to 5 carbon atoms,preferably methyl or tert-butyl, or alkoxy, for example of 1 to 5 carbonatoms, preferably methoxy, and X.sup.⊖ is AsF₆.sup.⊖, SbF₆.sup.⊖,PF₆.sup.⊖, BF₄.sup.⊖, ClO₄.sup.⊖, CH₃ SO₃.sup.⊖ or CF₃ SO₃.sup.⊖(=triflate).

Examples of particularly suitable components (b) are triphenylsulfoniumsalts and diphenyliodonium salts, tris-(4-hydroxyphenyl)-sulfoniumsalts, tris-(trimethylsilyloxyphenyl)-sulfonium salts,tris-(4-tert-butoxycarbonyloxyphenyl)-sulfonium salts andtris-(4-ethoxycarbonyloxyphenyl)-sulfonium salts, each havingClO₄.sup.⊖, AsF₆.sup.⊖, PF₆.sup.⊖, SbF₆.sup.⊖, BF₄.sup.⊖, CH₃ SO₃.sup.⊖or CF₃ SO₃.sup.⊖ as the counter-ion X.sup.⊖.

Mixtures of the compounds stated under (b) can also be used. Component(b) is present in the novel radiation-sensitive mixture in general inamounts of from 1 to 20, preferably from 3 to 10, % by weight, based onthe total amount of radiation-sensitive mixture (a)+(b) (c).

c) According to the invention, organic compounds of the general formula(I) ##STR12## where R¹ and R² are identical or different and are eachhydrogen, alkyl, for example of 1 to 5 carbon atoms, preferably methylor tert-butyl, alkoxy, for example of 1 to 5 carbon atoms, preferablymethoxy, aryl, e.g. phenyl, or aralkyl, e.g. benzyl, and R³ and R⁴ areidentical or different and are each alkyl, for example of 1 to 5,preferably 2 to 4, carbon atoms, e.g. ethyl, n-propyl or n-butyl,aralkyl where the alkyl group is of 1 or 2 carbon atoms, such as benzyl,halogen-substituted aralkyl, e.g. p-chlorobenzyl, aryl, e.g. phenyl,alkyl-substituted aryl, such as tolyl, alkoxy-substituted aryl, such asmethoxyphenyl, aryl which is substituted by halogen (such as F, Cl, Bror I, preferably Cl or Br), e.g. p-chlorophenyl, or a radical ##STR13##where n is from 1 to 5, preferably 1, and R⁵ is alkyl of, for example, 1to 5 carbon atoms, preferably one carbon atom, alkoxy of, for example, 1to 5, preferably 1 to 4, carbon atoms, e.g. methoxy, ethoxy ortert-butoxy, aryl, e.g. phenyl, or aralkyl, for example having 1 or 2carbon atoms in the alkyl group, such as benzyl, or R³ together with R⁴form a five-membered to seven-membered, preferably six-membered, ringvia --(CH₂)_(m) -- in which m is from 4 to 6, preferably 5, are used asorganic compounds (c) which suppress the solubility of the binder (a).

Preferred organic compounds (c) are ##STR14## where R' and R" areidentical or different and are each alkyl of 1 to 6, preferably 2 to 4,carbon atoms, phenyl or benzyl.

Examples of preferred organic compounds (c) are: ##STR15##

Mixtures of the abovementioned organic compounds (c) can also be used.Organic compounds (c) can be prepared by known processes, as describedin, for example, Protective Groups in Organic Synthesis, page 108 etseq., for example from pyrocatechol and carbonyl compounds, e.g.cyclohexanone, in the presence of an acid.

Component (c) is present in the novel radiation-sensitive mixture ingeneral in amounts of from 1 to 49, preferably 10 to 27, % by weight,based on the total amount of components (a)+(b)+(c).

The novel radiation-sensitive mixture may additionally contain furtherconventional assistants and additives.

The novel mixtures are preferably dissolved in an organic solvent, thesolids content generally being from 5 to 40% by weight. Preferredsolvents are aliphatic ketones, ethers and esters, and mixtures thereof.Alkylene glycol monoalkyl ethers, for example ethylcellosolve,butylglycol, methylcellosolve and 1-methoxy-2-propanol, alkylene glycolalkyl ether esters, for example methylcellosolve acetate,ethylcellosolve acetate, methylpropylene glycol acetate andethylpropylene glycol acetate, ketones, for example cyclohexanone,cyclopentanone and methyl ethyl ketone, and acetates, such as butylacetate, and aromatics, such as toluene and xylene, are particularlypreferred. The choice of the corresponding solvents and mixtures thereofdepends on the choice of the particular phenolic polymer, of the novolakand of the photosensitive component.

Other additives, such as adhesion promoters, wetting agents, dyes andplasticizers, may also be introduced, in general in amounts of not morethan 1% by weight.

If required, small amounts of sensitizers may also be added in order tosensitize the compounds in the longer-wavelength UV range up to thevisible range. Polycyclic aromatics, such as pyrene and perylene, arepreferred for this purpose, but other dyes which act as sensitizers mayalso be used.

In the novel process for the production of relief images, aradiation-sensitive recording layer which essentially consists of thenovel radiation-sensitive mixture is exposed imagewise to a dose suchthat the solubility of the exposed parts in aqueous alkaline solventsincreases and these exposed parts can be selectively removed by means ofthe alkaline developer.

Particular advantages of the novel radiation-sensitive mixtures are verygood reproducibility, high resolution and high sensitivity in theshort-wavelength UV range.

The photoresist solutions containing the novel radiation-sensitivemixture are generally applied in layer thicknesses of from 0.1 to 5 μm,preferably from 0.5 to 1.5 μm, to suitable substrates, forsurface-oxidized silicon wafers, by spin coating, and the layers aredried (for example at from 70° to 130° C.) and are exposed imagewise toa suitable light sources through a photomask. Particularly suitablelight sources are short-wavelength UV lamps (deep UV) having wavelengthsof from 200 to 300 nm and excimer lasers of KrF (248 nm). After theimagewise exposure and if necessary after a brief postbake at not morethan 150° C, development is carried out using a conventional aqueousalkaline developer, in general at a pH of from 12 to 14, the exposedparts being washed out. The resolution is in the submicron range. Theexposure energy required for the novel radiation-sensitive mixtures isin general from 80 to 250 mJ/cm² for a layer thickness of 1 μm.

In the Examples which follow, parts and percentages are by weight,unless stated otherwise.

EXAMPLE 1

A photoresist solution is prepared from 80 parts of apoly-(p-hydroxystyrene) having a mean molecular weight M_(w) of 7,000, 5parts of tris-(4-hydroxyphenyl)sulfonium triflate, 15 parts of thecompound ##STR16## and 250 parts of ethylene glycol monomethyl etheracetate.

The solution is then filtered through a filter having a pore diameter of0.2 μm.

The resist solution is applied by spin coating at 4000 rpm for 30seconds to a silicon wafer which is coated with hexamethyldisilazane asan adhesion promoter, layer thickness of about 1 μm being obtained. Thewafer is dried on a hotplate at 80° C. for 3 minutes, then brought intocontact with an imagewise structured test mask and exposed to an excimerlaser (λ=248 nm, E=35 mW/cm²). Thereafter, the wafer is heated at 80° C.for 1 minute and is developed with a developer of pH 12.0-13.6. Thephotosensitivity is 150 mJ/cm².

EXAMPLE 2

A photoresist solution is prepared by mixing 70 parts ofcresol/formaldehyde novolak (M_(w) : about 300), 5 parts oftriphenylsulfonium hexafluoroarsenate (commercial product from ALFA), 25parts of the compound ##STR17## and 250 parts of ethylene glycolmonomethyl ether acetate, and is further processed as described inExample 1. A photosensitivity of 190 mJ/cm² is obtained.

EXAMPLE 3

A photoresist solution comprising 70 parts of poly-(p-hydroxystyrene)(M_(w) : 7,000), 5 parts of triphenylsulfonium hexafluoroarsenate, 25parts of the compound ##STR18## and 250 parts of ethylene glycolmonomethyl ether acetate, is further processed similarly to Example 1. Aphotosensitivity of 80 mJ/cm² is obtained.

We claim:
 1. A radiation-sensitive mixture, consisting essentially ofeffective amounts of(a) a binder or binder mixture which is insoluble inwater but soluble in aqueous alkaline solutions, (b) a compound whichforms a strong acid on exposure to radiation and (c) one or more organiccompounds which suppress the solubility of (a) in aqueous alkalinesolutions, compound (c) is a compound of the general formula (I)##STR19## where R¹ and R² are identical or different and are eachhydrogen, alkyl, alkoxy, aryl or aralkyl, R³ and R⁴ are identical ordifferent and are each alkyl, cycloalkyl, aralkyl, unsubstituted oralkyl- or halogen-substituted aryl or a radical ##STR20## where n isfrom 1 to 5 and Rs is alkyl, alkoxy, aryl or aralkyl, or R³ togetherwith R⁴ forms a five-membered to seven-membered ring via --(CH₂)_(m) --in which m is from 4 to
 6. 2. A radiation-sensitive mixture as claimedin claim 1, wherein the binder (a) used is a phenolic resin.
 3. Aradiation-sensitive mixture as claimed in claim 2, wherein the phenolicresin used is a novolak having a mean molecular weight M_(w) of from 300to 20,000.
 4. A radiation-sensitive mixture as claimed in claim 1,wherein the binder (a) used is poly-(p-hydroxystyrene),poly-(p-hydroxy-α-methylstyrene) or a copolymer of p-hydroxystyrene andp-tert-butoxycarbonyloxystyrene, or p-hydroxystyrene and alkoxstyrene orof p-hydroxystyrene and 2-tetrahydropyranyloxystyrene (where thesecopolymers can also be prepared by a polymer-analogous reaction), eachhaving a mean molecular weight M_(w) of from 200 to 200,000.
 5. Aradiation-sensitive mixture as claimed in claim 1 wherein the bindercomponent (a) is present in an amount of from 50 to 95% by weight basedon the total amount of components (a)+(b)+(c), with the proviso that thesum of the percantages stated under (a), (b) and (c) is
 100. 6. Aradiation-sensitive mixture as claimed in claim 1, wherein an onium saltof the formula (II) or (III) ##STR21## where R.sup.α, R.sup.β andR.sup.γ are identical or different and are each alkyl, aryl, aralkyl ora radical ##STR22## where R.sup.δ, R.sup.ε and R.sup.ξ are identical ordifferent and are each H, OH, halogen, alkyl or alkoxy and X.sup.⊖ isAsF₆.sup.⊖, SbF₆.sup.⊖, PF₆.sup.⊖, BF.sup.⊖, ClO₄.sup.⊖, CH₃ SO₃.sup.⊖or CF₃ SO₃.sup.⊖, is used as the compound (b) which forms a strong acidon exposure to radiation.
 7. A radiation-sensitive mixture as claimed inclaim 1, wherein component (b) is prasant in an amount of from 1 to 20%by weight, based on the totaI amount of components (a)+(b)+(c).
 8. Aradiation-sensitive mixture as claimed in claim 1, wherein component (c)is prssent in an amount of from 1 to 49% by weight, based on the totalamount of components (a)+(b)+(c).
 9. A radiation-sensitive mixture asclaimed in claim 1, which additionally contains a sensitizer whichabsorbs radiation and transfers it to component (b).
 10. Aradiation-ssnsitive mixture as claimed in claim 1, whioh additionallycontains not more than 1% by weight, based on the totaI amount ofcomponents (a)+(b)+(c), of an adhesion promotsr, a surfactant or a dye.